A facile and green strategy for the synthesis of 1-dimensional luminescent ZnO nanorods and their reduction behavior for aromatic nitro-compounds

Abstract

This article illustrates a facile microwave assisted synthesis of 1D ZnO nanorods using lauric acid. A fatty acid acts as a complexing and capping agent in the synthesis of the ZnO nanorods. The average diameter of the ZnO nanorods was ∼5.5–10.0 nm. The ZnO nanorods were characterized using TEM, SAED, XRD, FTIR, UV and PL techniques. The synthesized ZnO nanorods showed unusual luminescence properties. For the first time, the reduction of toxic aromatic nitro compounds, such as para-nitrophenol, para-nitroaniline and 2,4,6-trinitrophenol was carried out using the synthesized ZnO nanorods as a catalyst in the presence of NaBH₄ in aqueous medium.

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1. Introduction

Aromatic nitro-compounds are the most toxic organic compounds. para-Nitrophenol is one of the hazardous nitro-compounds and it causes severe health hazards which impose a threat to human, animal and aquatic lives. Nitro compounds bearing more than one nitro group were generally used as explosives such as, 2,4,6-trinitrophenol, which was widely used as an explosive in wars and industries. The presence of a strong electron withdrawing nitro-group, which is capable of inducing explosive properties in aromatic compounds, is called an explosophore.¹ Hence, their disposal into nearby water sources from industries causes serious environmental pollution and harms the ecosystem. Various effective means were developed for the treatment of such toxic compounds. The reduction of nitro compounds into their amino analogues is one of the most effective means for the treatment of such toxic compounds. The amino derivatives were non-toxic and they were used as important intermediates in the synthesis of various analgesic and antipyretic drugs, and in the synthesis of various dyes, as photographic-developers and corrosion inhibitors.² Therefore, it is more suitable to develop an effective method to convert the nitro compounds into their amino derivatives. The conventional methods of the hydrogenation of nitro compounds were associated with serious environmental problems. In recent years, much attention was devoted towards the catalytic reduction of aromatic nitro compounds using NaBH₄ in the presence of noble metal nanoparticles as a catalyst.³ However, there are much fewer reports regarding the catalytic activity of metal oxide nanoparticles in the reduction of aromatic nitro-compounds. Therefore, in this paper, for the first time, we report the catalytic reduction of aromatic nitro-compounds, such as para-nitrophenol (PNP), para-nitroaniline (PNA) and 2,4,6-trinitrophenol (TNP) using NaBH₄ in the presence of ZnO nanorods as the catalyst, in aqueous medium.

ZnO, a II–VI semiconductor having a wide band gap of 3.37 eV, has gained a lot of interest in recent years because of its fascinating luminescence, optical, electrical, chemical and biological properties. ZnO nanoparticles have a wide range of promising applications in catalysis, antibacterial treatment, the fabrication of sensors and solar cells, optoelectronic devices namely LED and lasers, etc.^4–6 Numerous methods and techniques were employed for the synthesis of ZnO nanoparticles with various morphologies.^5–7 However, most of the synthesis methods were associated with high temperature, expensive reagents, typical reaction conditions, sophisticated instruments and tedious reaction strategies. In this paper, we report a facile microwave heating method for the synthesis of 1D ZnO nanorods (NRs). This communication describes the lauric acid mediated synthesis of ZnO NRs in ethanol. The synthesized ZnO NRs show enhanced luminescence properties. Many researchers have reported ZnO nanoparticles having blue and green emission bands.⁷ However, this paper reports yellow emission observed in the case of the synthesized ZnO NRs which is very rare, along with negligible blue and green emission.

2. Experimental

Materials and method

All the reagents, zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), methylamine, lauric acid, NaBH₄, p-nitrophenol, p-nitroaniline and 2,4,6-trinitrophenol were procured from Merck and were of analytical grade (AR). The reaction was carried out in a domestic microwave oven.

The ZnO NRs were synthesized using zinc nitrate hexahydrate, lauric acid and methyl amine. In this experiment, 0.01 M zinc nitrate hexahydrate was dissolved in 20 ml of ethanol and was treated with 20 ml of a 0.01 M ethanolic solution of lauric acid and 10 ml of 40% methyl amine aqueous solution with constant stirring. The pH of the reaction was maintained at 11. The reaction mixture was then irradiated with microwaves for 180 s. This leads to the formation of a white precipitate which was centrifuged and washed with alcohol and distilled water several times. The precipitate was finally calcined at 120 °C and collected for characterization.

The ZnO NRs were characterized using a powder X-ray diffraction (XRD) method using a Phillips X’Pert PRO diffractometer with CuKα radiation of wavelength 1.5418 Å. The size, morphology and diffracted ring pattern of the ZnO NRs were determined using a JEM-2100 Transmission Electron Microscope. The infrared spectrum was recorded using a Bruker Hyperion 3000 FTIR spectrometer. The absorption spectra were recorded using a Cary 100 BIO UV-visible spectrophotometer.

Catalytic reduction of toxic aromatic nitro compounds

For the reduction of aromatic nitro compounds, a 6 mM solution of the aromatic nitro compound (p-nitrophenol, p-nitroaniline and 2,4,6-trinitrophenol) and a 0.1 M solution of NaBH₄ were prepared. All the reactions were carried out in a quartz cuvette and the absorbance was recorded in order to investigate the reduction process. In the cuvette, 2 ml of water was taken and then 60 μl of the nitro compound and 350 μl of NaBH₄ solution were added and the absorption was recorded. To the above solution, 300 μl of the solution of the ZnO NRs (0.005 g) was added to initiate the reduction process. The absorbance was recorded immediately and at a regular interval of time to study the reduction of the aromatic nitro-compounds.

3. Results and discussion

The FTIR spectrum was recorded in order to investigate the formation of ZnO and the role of lauric acid in the synthesis pathway. Fig. 1a represents the FTIR spectrum of the synthesized ZnO NRs. The band observed at 497 cm⁻¹ was attributed to the characteristic vibration of a Zn–O bond.⁸ This peak confirmed the formation of the Zn–O bond. The appearance of a broad band around 3515 cm⁻¹ corresponds to the OH group of lauric acid. The presence of lauric acid was also confirmed by the occurrence of sharp peaks at 2925 and 2854 cm⁻¹ due to C–H asymmetric and symmetric stretching vibration. Therefore, lauric acid gets adsorbed on the surface of the ZnO NRs. It was also evident that the band around 1710 cm⁻¹ observed in the case of lauric acid was absent. The existence of bands around 1570 and 1415 cm⁻¹ correspond to a carboxylate asymmetric and symmetric stretch. This confirmed the existence of lauric acid as a laurate anion absorbed on the surface of ZnO. Therefore, the analysis of the FTIR spectrum suggests that lauric acid acts as a capping agent in the synthesis of the ZnO NRs.

Fig. 1 (a) FT-IR spectrum of the synthesized ZnO NRs showing the formation of ZnO and analyzing the role of lauric acid as a capping agent in the synthesis of the ZnO NRs, (b) XRD pattern of the synthesized ZnO NRs predicting a hexagonal crystalline structure, (c) absorption spectrum of the synthesized ZnO NRs showing a broad absorption band around 379 nm (upper right inset represents the corresponding plot of (αhν)² versus photon energy, hν) and (d) PL spectrum of the as-prepared ZnO nanorods (λ_ex = 300 nm).

Fig. 1b represents the XRD pattern of the synthesized ZnO NRs. The crystalline structure, nature and purity of the compound were investigated using the XRD pattern. The occurrence of peaks at 2θ values of 31.86, 34.55, 36.33, 47.65, 56.69, 62.89, 66.33, 68.1, 69.13, 72.9, 77.14 and 89.71 °C clearly reflects the (100), (002), (101), (102), (110), (103), (200), (112) (201), (004), (202) and (203) planes, respectively. The diffraction pattern was well indexed to the hexagonal crystal structure of ZnO and was also in good agreement with JCPDS card no. 89-0511. The appearance of sharp peaks depicts that the synthesized material is well crystallized. The diffraction pattern did not show any peaks for the precursor molecules or impurities which indicated the purity and complete conversion of the starting material into products. The average crystalline size of the ZnO NRs obtained using the Debye–Scherrer equation⁶ was 18.08 nm.

The optical properties of the ZnO NRs were investigated using the UV and PL spectra. The UV-visible spectrum (Fig. 1c) shows a broad absorption band at around 379 nm which confirms the formation of the ZnO nanoparticles and is associated with the transfer of electrons from the valence band to the conduction band.⁸ The optical band gap energy can be obtained from the absorption onset using a Tauc’s plot.⁶ The band gap energy of the synthesized ZnO nanoparticles can be obtained from the following equation:⁶

αhν = K(hν − E_g)ⁿ (1)

where K is a constant, E_g is the band gap energy and the exponent ‘n’ depends on the type of transition whose value is 1/2 for an allowed direct transition. Therefore, by plotting (αhν)² versus incident photon energy (hν) and by extrapolating the linear portion of the curve to the zero absorption coefficient, the band gap energy can be estimated. The inset of Fig. 1c shows the plot of (αhν)² versus (hν) for the synthesized ZnO nanorods. The band gap energy of the ZnO NRs was calculated from the plot of (αhν)² versus photon energy (hν) (inset of Fig. 1c) by extrapolating the linear region of the curve to the zero absorption coefficient and was found to be 3.28 eV.⁶

The PL spectrum gives brief information regarding the purity and quality of the synthesized material. The room temperature PL spectrum (Fig. 1d) of the ZnO NRs shows a strong UV emission band centered at 387 nm and an emission band at 583 nm when λ_ex = 300 nm. The UV emission at 387 nm is band-edge emission which arises due to the recombination of free excitons between the valence and conduction bands.⁶ The yellow emission band at 583 nm occurs due to native deep level defects mainly attributed to oxygen interstitials (O_i⁻).⁹ Blue and green emission bands of very negligible intensity were also observed around 450 and 550 nm, respectively. The occurrence of blue and green emission peaks was due to the radiative transition from the extended Zn_i to the valence band and from the conduction band to the deep oxygen vacancy level, respectively.⁷ The intensity of the UV emission reveals the crystallinity of the sample and the intensity of the bands in the visible region reflects the concentration of defects in the sample.¹⁰ Hence, the PL spectrum indicates the highly crystalline nature of the synthesized ZnO NRs and the presence of defects also indicates an increase in the optical and electrical properties of the synthesized material. The synthesized ZnO nanorods showed unusual luminescence properties, thereby showing a prominent yellow emission band. Roy et al. synthesized ZnO nanorods with an average length and diameter of 100–150 nm and 10–20 nm, respectively (aspect ratio = 10), which showed blue and green emission bands.⁷ The dependence of the luminescence properties on the change in the size of ZnO nanoparticles was studied by Raoufi and in that case, he concluded that with an increase in particle size, the intensity of visible deep level emission increases.¹¹ Wang et al. also showed that the photoluminescence properties of ZnO nanoparticles are size-dependent.¹² Herein, we report the synthesis of ZnO nanorods with a lower aspect ratio (length : diameter) of 1.3–1.8 which showed unusual yellow emission along with blue and green emission bands. Therefore, the appearance of an unusual yellow emission band may be attributed to the lower aspect ratio of the synthesized ZnO nanorods. Hence, the incorporation of lauric acid leads to a decrease in the length and diameter of the ZnO nanorods thereby decreasing the aspect ratio which plays an important role in determining the luminescence properties of the ZnO nanoparticles.

The energy band diagram shown in Scheme 1 explains the probable mechanism of blue, green and yellow emission in the ZnO nanorods. The mechanism of blue, green and yellow emission bands was related to the coexistence of Zn_i, oxygen vacancies and O_i in the synthesized ZnO nanorods. The electrons from the valence band get excited to the conduction band only when the energy of the incident photon is greater than the band gap of the synthesized ZnO nanorods. The synthesized ZnO NRs showed blue emission having energy of 2.75 eV which was due to the transition from the extended Zn_i to the valance band. The calculated energy gap between the conduction band and the Zn_i was found to be 0.53 eV. The electrons excited from the valence band to the conduction band undergo a non-radiative transition to the extended Zn_i, wherein a further transition occurs directly to the valence band giving rise to the blue emission band at 450 nm. The electrons that were forced into the conduction band de-excite to the deep levels of the oxygen vacancies and this gives rise to the green emission band at 550 nm. The calculated energy for green emission was 2.25 eV.

Scheme 1 Schematic diagram for the probable mechanism of blue, green and yellow transitions.

The synthesized ZnO nanorods showed unusual luminescence properties by giving rise to a prominent yellow emission band along with negligible blue and green emission bands. The origin of yellow emission is attributed to oxygen interstitials, O_i. The calculated yellow emission energy for the synthesized ZnO NRs was 2.11 eV. The mechanism of yellow emission proceeds through the non-radiative transition of excited electrons from the conduction band to the extended oxygen interstitial O_i, with an energy difference of 1.17 eV. From the O_i, the electrons undergo a further transition to the valence band thereby giving rise to an intense yellow emission band at 583 nm. The intensity of the blue and green emission bands is negligible in comparison to that of the yellow emission band which indicates that most of the excited electrons from the conduction band undergo a non-radiative transition to the extended oxygen interstitials, O_i and then further undergo a transition to the valence band thereby giving rise to a prominent high intensity yellow emission band.

The morphology and particle size distribution of the synthesized ZnO nanoparticles were elucidated using Electron Microscopic analysis. The TEM image (Fig. 2a) shows the formation of rod-like ZnO nanoparticles with a diameter and length of 5.5–10.0 and 10–13 nm, respectively. The microstructure of the ZnO nanorods was analyzed using the HRTEM image (Fig. 2b). The spacings between the adjacent lattice planes obtained from the HRTEM image (Fig. 2b) are 0.122 and 0.165 nm which corresponded to the (202) and (110) lattice planes, respectively. The SAED pattern (Fig. 2c) revealed the hexagonal crystal structure of the ZnO NRs which resembled the XRD pattern. The SAED pattern also reflects that the ZnO NRs were well crystallized in nature.

Fig. 2 (a) TEM image of the as-prepared ZnO nanorods having an average diameter and length of 5.5–10.0 nm and 10–13 nm respectively, (b) HRTEM image of the synthesized ZnO nanorods and (c) SAED pattern of the synthesized ZnO nanorods having a hexagonal crystal structure.

In the synthesis of the ZnO NRs, lauric acid plays the role of a complexing as well as a capping agent. By being basic in nature, methyl amine is supporting the deprotonation of lauric acid thereby enhancing the formation of the ZnO NRs. However, for further confirmation of the role of lauric acid as a capping agent in the synthesis of the ZnO NRs, a control experiment was carried out for the synthesis of the ZnO nanoparticles in the absence of lauric acid and the corresponding TEM image and FTIR spectrum were recorded (Fig. S1a and b†). The TEM image (Fig. S1a†) shows the formation of the ZnO nanorods with an average diameter and length of 130 nm and 415 nm, respectively. This clearly indicates that the involvement of lauric acid in the synthesis of the ZnO nanoparticles leads to the formation of small sized ZnO nanorods. The adsorption of laurate anions on the surface of the ZnO nanorods terminates the growth of the ZnO nanoparticles and prevents agglomeration by giving rise to small sized ZnO nanorods with diameters and lengths of 5.5–10 nm and 10–13 nm, respectively. The FTIR spectrum (Fig. S1b†) of the ZnO nanoparticles synthesized in the absence of lauric acid was also recorded and the spectrum showed a band at around 491 cm⁻¹. The appearance of this band confirmed the formation of a Zn–O bond. No bands were observed in the region of 1700–1500 cm⁻¹ which clearly depicts the absence of the laurate anions on the surface of the ZnO nanorods which inhibit the growth of the ZnO nanorods thereby acting as a good capping agent.

Reduction of toxic aromatic nitro compounds using the ZnO NRs as a heterogeneous catalyst

The catalytic reduction of aromatic nitro-compounds using NaBH₄ was taken as a model reaction to evaluate the catalytic activity of the ZnO nanorods. Fig. 3a presents the absorption spectra which indicate the change of concentration of p-nitrophenol (PNP) with time in aqueous medium in the presence of NaBH₄ acting as a reducing agent and the ZnO NRs as a heterogeneous catalyst. It was evident that the absorption maximum of PNP was centered at 317 nm and after the addition of NaBH₄ the absorption maximum shifted to 403 nm due to the formation of p-nitrophenolate ions (Fig. S2†). The color of the solution also changes to intense yellow after the addition of NaBH₄ due to the formation of the p-nitrophenolate ions under alkaline conditions. The peak at 403 nm remains unaltered after a couple of days in the absence of any catalyst. It is a kinetically restricted reaction in the absence of any catalyst. Therefore, PNP reduction doesn’t take place using only the reducing agent NaBH₄ in the absence of any catalyst.² However, after the addition of the ZnO NRs the absorption maximum at 403 nm gradually decreases with time which indicates the reduction of PNP and the color of the solution also fades away. A new peak centered at 300 nm started to be generated with an increase in the reduction time. This peak arises due to the formation of p-aminophenol which is the final reduced product.^2,3 The complete reduction takes place within 120 min. The rate of the reaction was studied using first order kinetics by plotting ln A_t versus time (Fig. 3b) and was found to be 0.026 min⁻¹.

Fig. 3 (a) Absorption spectra of the conversion of PNP to PAP using the ZnO nanorods as catalyst and (b) plot of ln[A_t] versus reduction time.

The absorption spectrum for the reduction of p-nitroaniline (PNA) using NaBH₄ in the presence of the ZnO NRs as the heterogeneous catalyst is presented in Fig. 4a. The disappearance of the peak at 380 nm and the appearance of a new peak centered at 300 nm indicate the complete reduction of PNA to p-phenylenediamine.³ The rate of the reaction was obtained using first order kinetics (Fig. 4b) and the rate constant was found to be 0.044 min⁻¹. Similarly, the reduction of 2,4,6-trinitrophenol (TNP) was carried out using NaBH₄ as a reducing agent in the presence of the ZnO NRs acting as a catalyst. From the UV-visible spectrum (Fig. 5a), it is evident that TNP showed an absorption maximum at 358 nm. The disappearance of the band at around 357 nm and the appearance of a new peak at 300 nm showed the complete reduction of the nitro compound into its corresponding amino derivative.³ This reaction also followed first order kinetics. The rate of the reaction obtained from the linear plot of ln A_t versus time (Fig. 5b) was 0.141 min⁻¹.

Fig. 4 (a) UV-spectra of the reduction of PNA using NaBH₄ in the presence of the ZnO nanorods as catalyst and (b) plot of ln A_t versus reduction time.

Fig. 5 (a) UV-spectra of the reduction of TNP using NaBH₄ in the presence of the ZnO nanorods as catalyst and (b) plot of ln A_t versus reduction time.

Moreover, to verify the role of the ZnO NRs and NaBH₄, control experiments were also performed for the reduction of PNP, PNA and TNP using only NaBH₄ in the absence of the ZnO NRs. It was found that all the aromatic nitro-compounds were not reduced even after a couple of days. The reduction took place only when the ZnO NRs were added. Moreover, the reduction of the nitro compounds carried out using the ZnO NRs in the absence of NaBH₄ doesn’t take place successfully which further confirmed that the ZnO NRs cannot act as a reducing agent. These control experiments confirmed the role of NaBH₄ as a reducing agent and the ZnO NRs as a catalyst in the effective reduction of aromatic nitro compounds.

Therefore, the ZnO NRs act as an effective catalyst in the reduction of p-nitrophenol, p-nitroaniline and 2,4,6-trinitrophenol to their corresponding amino derivates. Table 1 presents the overall study for the reduction of toxic aromatic nitro compounds in aqueous medium using the ZnO NRs as a catalyst.

Table 1 ZnO nanorod catalyzed reduction of toxic aromatic nitro compounds in aqueous medium

Entry	Substrate	Product	Time (min)	Rate constant, k (min^−1)
1			120	0.026
2			45	0.044
3			18	0.141

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4. Conclusion

A facile microwave assisted route was developed for the synthesis of 1D ZnO NRs using lauric acid as a complexing and capping agent in ethanol medium. The ZnO NRs, having an average diameter of 5.5–10.0 nm, possess a hexagonal crystal structure and are well crystalline in nature. The ZnO NRs showed unusual luminescence properties wherein, high intensity UV and yellow emission bands were observed along with negligible blue and green emission bands. Toxic nitro-compounds, such as PNP, PNA and TNP were completely reduced into their amino derivatives using NaBH₄ in the presence of the ZnO NRs as a catalyst in aqueous medium within 120, 45, and 18 min, respectively.

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Footnote

† Electronic supplementary information (ESI) available: TEM image and FTIR spectrum of ZnO nanoparticles synthesized in absence of lauric acid, the absorption spectrum of PNP and absorption spectrum of PNP after the addition of NaBH₄. See DOI: 10.1039/c5ra22908a

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